Method for solidifying radioactive waste

ABSTRACT

A process for solidifying radioactive wastes, which comprises adding, to pellet or dissolved radioactive wastes, an alkali metal silicate as a filler, silicic acid, carbonic acid or an alkaline earth metal or polyvalent metal salt thereof as a hardener, and cement as an absorbent for absorbing water to be released with the progress of the reaction and, if necessary, water and mixing and solidifying the resulting mixture to thereby prevent deterioration of water-proofness due to precipitation of readily soluble salts on the surface of a solidified substance.

TECHNICAL FIELD

This invention relates to a method for solidifying radioactive wastegenerated, for example, in a nuclear power station and more particularlyto a method for solidifying radioactive waste which is especiallyeffective when an alkali silicate or an aqueous solution thereof is usedas solidifying filler.

TECHNICAL BACKGROUND

A way of final disposal of radioactive waste is retrievable storage orground disposal wherein the radioactive waste has to be treated to givea solidified waste constructed of a solidified body for this purpose.Cement has been used as the solidifying filler in forming the solidifiedradioactive waste, but lately there was developed an alkali silicate(solution), as a replacement of cement, which is most suited for thesolidification and disposal of the pelletized radioactive waste withhigh volume, reduction (see Japanese laying-open patent publication No.57-197,500(1982) published on Dec. 3, 1982).

It has been confirmed that when radioactive waste is treated into asolid body by using a solidifying agent prepared by mixing the alkalisilicate (solution) as the solidifying filler, an inorganic phosphatecompound (P₂ O₅ :SiO₂) as hardening agent and cement as a waterabsorbent, the formed solid body has favorable properties, such as highstrength, high heat resistance, good durability, and the like. It wasfound, however, that easily soluble salts are deposited on the surfaceof the solidified body after hardened. In the course of the solidifyingprocess, the solidifying agent undergoes reactions represented by thefollowing formulae (1) and (2):

    M.sub.2 O.nSiO.sub.2.xH.sub.2 O+P.sub.2 O.sub.5.SiO.sub.2    nSiO.sub.2 +M.sub.3 PO.sub.4 +xH.sub.2 O                             (1)

    CaO.SiO.sub.2 +xH.sub.2 O   CaO.SiO.sub.2.xH.sub.2 O       (2)

wherein M represents an alkali metal.

The above formulae (1) and (2) correspond to the hardening of the alkalisilicate solution by the inorganic phosphate compound and the absorptionof the generated water by cement, respectively. The salt M₃ PO₄(actually a mixture of M₂ HPO₄, MH₂ PO₄, M₂ H₂ P₂ O₇, M₃ PO₄ and theirhydrates) produced in the hardening reaction (1) is an easily solublematter, with its solubility being about 30 wt%, and is dissolved in theliberated water generated in the course of the hardening reaction (1).This dissolving reaction is competitive with the water absorbingreaction, but the former reaction advances faster than the latter sinceboth the salt and the liberated water are formed in the same reaction.The undissolved portion of the salt remains in the hardened solidifiedbody, but the dissolved salt migrates within the solidified body. Thus,as the phenomena observed in the hardened solidified body, there takesplace the migration of the liberated water (solution) in which said salthas been dissolved and the evaporation of water from the solidified bodysurface. Consequently, the liberated water is shifted toward thesolidified body surface due to the capillary action and is evaporatedtherefrom, leaving the recrystallized salt on the solid body surface.This accounts for the phenomenon of salt deposition.

The deposited salt, which is easily soluble as mentioned before,deteriorates the water resistance of the solidified radioactive waste,inviting the danger of causing leakage of radioactive nuclides into theenvironment. The salt deposition caused when the solidified body made byusing an inorganic phosphate compound as hardening agent was left in aroom and the alkali metal elution observed when the solidified body wasimmersed in water are shown by curves (A) of FIGS. 1 and 2,respectively. As seen from these curves (A), when an inorganic phosphatecompound is used as hardening agent, approximately 1% by weight of saltis deposited when the solidified body is left in a room for 400 hoursand approximately 8% by weight of alkali metal is eluted when saidsolidified body is immersed in water for the same period of time.

Thus, the prior art method using an inorganic phosphate compound (P₂O₅.SiO₂) as the hardening agent of the alkali silicate solidifyingfiller was attended by the problem that the salts are deposited on thesurface of the produced solidified body of radioactive waste because ofthe formation of easily soluble salt M₃ PO₄ in the hardening reaction,and the deposition of salts causes the deterioration of the waterresistance of the solidified body which might lead to the leakage ofradioactive nuclides from the solidified body.

DESCRIPTION OF THE INVENTION

An object of this invention is to provide a method for forming asolidified radioactive waste having high strength, heat resistance anddurability as well as excellent water and moisture resistance by using aspecific solidifying agent so that the salt formed in the hardenedsolidified body of the radioactive waste is made a hardly soluble matter(with a solubility of below 5% by weight) to thereby prevent the saltfrom being deposited on the solidified body surface.

The method of this invention is to solidify radioactive waste by using asolidifying agent prepared by mixing an alkali silicate used assolidifying filler, a hardening agent for hardening said alkalisilicate, which hardening agent, used in place of the conventionalinorganic phosphate compound, is reacted with the alkali silicate toform a low-solubility salt, cement used as water absorbent, and waternecessary for mixing a material.

The hardening agent used in this invention is a compound containing abase which can combine with the alkali metal M in said alkali silicateto form a hardly soluble salt. The base usable in this inventionincludes TaO₃ ⁻, AlF₆ ³⁻, NbO₃ ⁻, SiF₆ ²⁻, SiO₃ ²⁻, BeF₄ ²⁻, B₄ O₇ ²⁻,F⁻, IO₄ ⁻, CO₃ ²⁻, ClO₄ ⁻, BF₄ ⁻, and ReO₄ ⁻. The solubility (% byweight) of the salts formed by these bases combined with the alkalimetals is shown in Table 1. In the table, mark"-" means unknown.

                  TABLE 1                                                         ______________________________________                                        Alkali metal                                                                  Base        Na.sup.+     K.sup.+                                                                              Li.sup.+                                      ______________________________________                                        TaO.sub.3.sup.-                                                                           0.01          0.01  0.02                                          AlF.sub.6.sup.3-                                                                          0.04         0.1    --                                            NbO.sub.3.sup.-                                                                           0.1          --     0.04                                          SiF.sub.6.sup.2-                                                                          0.67         1.1    --                                            SiO.sub.3.sup.2-                                                                          0.85         --     --                                            BeF.sub.4.sup.2-                                                                          1.7          1.4    12                                            B.sub.4 O.sub.7.sup.2-                                                                    2.6          --     --                                            F.sup.-     3.9          48     0.13                                          IO.sub.4.sup.-                                                                            9.3          0.4    --                                            CO.sub.3.sup.2-                                                                           18           53     1.3                                           ClO.sub.4.sup.-                                                                           66           1.7    36                                            BF.sub.4.sup.-                                                                            --           0.5    --                                            ReO.sub.4.sup.-                                                                           --           0.9    --                                            ______________________________________                                         (unit: wt % (at 20° C.))                                          

It was found that by using as hardening agent a compound containing abase which can meet the requirement of forming a salt with a solubilityof below 5% by weight and by using an alkali silicate as a solidifyingfiller, it is possible to prevent the deposition of any salt and to formthe desired solidified radioactive waste with high water resistance. Thehardening agent, that is, the compound containing a base capable ofmeeting said requirement is a compound between a polyvalent metal ionselected from the group consisting of Ca²⁺, Mg²⁺, Al³⁺ and Fe³⁺ or an H⁺ion and an ion selected from the group consisting of TaO₃ -, AlF₆ ³⁻,NbO₃ ⁻, SiFe²⁻, SiO₃ ²⁻, BeF₄ ²⁻, B₄ O₇ ²⁻, F⁻, IO₄ ⁻, CO₃ ²⁻, ClO₄ ⁻,BF₄ ⁻ and ReO₄ ⁻.

Curves (B) in FIGS. 1 and 2 show the results of actual measurement ofthe salt deposition rates when the solidified bodies obtained by usingCaCO₃, Ca(ClO₄)₂, CaSiF₆ and CaSiO₃ as examples of said hardening agentwere left in a room and the alkali metal elution rates when saidsolidified bodies were immersed in water. Although not shown in FIGS. 1and 2, it was also experimentally confirmed that the substantially sameresult can be obtained by using other hardening agents containing theabove-cited bases which can meet the above requirement.

All the bases usable in this invention are shown in Table 1, but SiO₃ ²⁻is the most preferred among them for the reasons following. Since SiO₂occurs in nature in abundance, it is expected that the use of SiO₃ ²⁻base would the conducive to good compatibility of the solidifiedradioactive waste with nature when disposing the solidified body innature, especially on land. Also, since SiO₂ is the principal componentof certain rocks such as granite which occur in nature stably throughthe order of hundreds of years, it is considered that the use of SiO₃ ²⁻base would make the solidified waste more durable than when using theother bases.

As for the metal ion to be combined with the base for forming thehardener, Ca²⁺ is the most preferred. This is because Ca²⁺ is moreeasily available at lower cost than other metal ions, and also, since itoccurs abundantly in nature, the solidified waste containing such ionhas a good compatibility with nature in ground disposal.

As viewed above, the problems of water resistance of the solidifiedwaste and especially the deposition of easily soluble salts on itssurface can be alleviated by using as hardening agent the compoundscontaining the bases as shown in Table 1. Another important evaluationfactor of the solidified radio-active waste is its strength. Thestrength is greatly influenced by the water content of the waste and thevoid ratio of the solidified waste. Therefore, the mixing ratios of thehardening agent, water absorbent and water, based on the ratio of thealkali silicate as the solidifying filler in the solidifying agent, willnow be discussed from the standpoints of the water content and the voidratio.

FIGS. 3 and 4 illustrate the relationship between the strength of thesolidified waste and the water content of the waste on one hand and thevoid ratio of the solidified waste on the other hand. These drawingsrepresent the case where CaSiO₃ was used as hardening agent, but thesimilar tendency is noted when other types of hardening agent mentionedabove are used. The waste originally (i.e. before solidification)contains about 3% by weight of water, and also the solidified wasteinvariably has at least about 10% of voids. In the graphs of FIGS. 3 and4, the ordinate refers to the relative strength as determined bynormalizing the strength under said conditions as 1.

It was found that the solidified waste becomes defective when itsrelative strength is below 0.5 (because of formation of cracks or otherproblems). Therefore, as noted from FIGS. 3 and 4, it is necessary toregulate the water content of the waste and the void ratio of thesolidified waste below about 6% by weight and about 30%, respectively.

It was also disclosed that the void ratio of the solidified waste dependon the viscosity of the solidifying agent before it is hardened. Thatis, if the solidifying agent has a high viscosity, the air entrappedtherein during stirring becomes sluggish in separating from thesolidifying agent (sol) before hardening, resulting in the increasedvoid ratio in the solidified waste. FIG. 5 shows the relationshipbetween the void ratio of the solidified waste and the viscosity (justafter the formation of sol) of the sol of the solidifying agent. Forreducing the void ratio below 30%, it needs to keep the viscosity of thesolidifying agent sol below 3,000 cP. Since the sol viscosity is easierto measure than the void ratio, the proper range of composition of thesolidifying agent can be decided from the two factors: the waterabsorption of the waste and the viscosity of the solidifying agent.

FIGS. 6, 7 and 8 show the results of examination of the water content ofthe waste and the viscosity of the solidifying agent by keeping themixing ratio of the alkali silicate filler constant (37.5% by weight)while changing the mixing ratios of the hardening agent, water absorbent(cement) and water. In these graphs, the amounts of the hardening agent,the cement, and the water added, respectively, are plotted as abscissaand the water content of the waste (on the left-hand vertical axis) andthe viscosity of the solidifying agent (on the right-hand vertical axis)as ordinate. From these graphs and the above-mentioned allowable rangesof the water absorption of the waste (below about 6% by weight) and theviscosity of the solidifying agent just after mixing (below about 3,000cP), it is found that the optimal ranges of the amounts of the hardeningagent to be added, the cement to be added and the water to be containedare 3 to 50% by weight, 3 to 35% by weight and 15 to 40% by weight,respectively.

By using a solidifying agent having the composition as specified above,it is possible to form a solidified radioactive waste having highmoisture and water resistance and substantially free of salt depositionas illustrated by curves (B) in FIGS. 1 and 2. From the experimentalresults shown in these figures, it is seen that in case of using thesolidifying agent of this invention as represented by the curves (B) interms of salt deposition rate, the obtained solidified waste is reducedin its salt deposition rate to more than 1/10 when it is left in a room,and when the solidified waste is immersed in water, the alkali metalelution rate is reduced to about 1/2, in comparison with the case ofusing an inorganic phosphate compound as hardening agent in which casethe salt deposition rate is as depicted by the curves (A). The ratherlimited improvement of the alkali metal elution rate is consideredattributable to the facts that the amount of alkali metal contained inthe alkali silicate used as the solidifying filler in the case of curves(B) is the same as in the case of curves (A) and that the amount ofwater used for the immersion is far greater (about 100 times) than thevolume of the solidified waste. In the case of the retrievable storagewhich is under the more mild conditions, the elution rate will bereduced to the level shown by the curves (B) of FIG. 1 and also thequality of the solidified waste will be greatly improved over the priorart.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the rate of salt deposition on the surface ofthe solidified waste with time when the waste was left in a room.

FIG. 2 is a graph showing the rate of alkali metal elution from thesolidified waste with time when the waste was immersed in water. Inthese graphs, curves (A) represent the prior art and curves (B)represent the embodiments of this invention.

FIGS. 3 and 4 are graphs showing the influence of the water content ofthe waste and the void ratio in the solidified waste, respectively, onthe relative strength of the waste.

FIG. 5 is a graph showing the relation between the void ratio of thesolidified waste and the viscosity of the solidifying agent.

FIGS. 6, 7 and 8 are graphs showing the relationship between the amountof hardening agent, cement and water, respectively, added in thesolidifying agent and water absorption of the waste and the viscosity ofthe solidifying agent.

FIGS. 9 and 10 are flow sheets illustrating the embodiments of themethod for solidifying radioactive waste according to this invention,where FIG. 9 shows the case where an aqueous solution of sodium silicatewas used as solidifying filler and FIG. 10 shows the case where powderedsodium silicate was used as the solidifying filler.

FIG. 11 is a diagrammatic drawing showing an example of the solidifiedwaste formed according to the method of this invention.

FIG. 12 is a flow sheet illustrating another embodiment of the methodfor solidifying radioactive waste according to this invention.

FIG. 13 is a diagrammatic drawing showing a homogeneous solidified wasteformed according to the embodiment of this invention shown in FIG. 12.

FIG. 14 is a flow sheet illustrating the method of forming a solidifiedradioactive waste according to the other embodiment.

FIG. 15 is a graph showing the amount of liberated water contained inthe solidified body and the evaporation rate of the liberated water as afunction of the degree of vacuum at the time of hardening.

PREFERRED EMBODIMENTS OF THE INVENTION

The embodiment illustrated in Figure pertains to a case where theradioactive waste to be treated is a concentrated liquid waste (mainlycomposed of Na₂ SO₄) generated in a nuclear reactor and where the wasteis dried, powdered and then pelletized, and by using a 60 wt% solutionof sodium silicate (Na₂ O.nSiO₂, n=0.5-4) as solidifying filler andcalcium silicate (CaSiO₃) as hardening agent, the pelletized waste issolidified in a 200-l drum.

First, as shown in FIG. 9, about 260 kg of pelletized radioactive waste7 mainly composed of Na₂ SO₄ is filled in a wire mesh basket 6 disposedin a 200-l drum 5. Then 150 kg of a 60 wt% solution of sodium silicate,60 kg of calcium silicate (CaSiO₃) and 30 kg of cement, contained intanks 1, 2 and 3, respectively, are supplied into a mixing stirrer 4 andhomogeneously mixed therein to form a solidifying agent, which is thenflown into the drum so that the solidifying agent fills the voidsbetween the pellets themselves and between the pellets and the drum.After filling, the mixture in the drum is deaerated under a vacuum ofabout 50 Torr to remove air bubbles remaining in the solidifying agentand then left at room temperature to allow the solidifying agent toharden. This hardening is completed in about 2 hours.

FIG. 10 depicts another embodiment of the invention where powderedsodium silicate is used in place of an aqueous solution of sodiumsilicate. In this case, in order to facilitate homogeneous mixing of thepowder and water, the powdered sodium silicate, powdered calciumsilicate, (CaSiO₃) and powdered cement contained in tanks 8, 2 and 3,respectively, are first supplied, in amounts of 90 kg, 60 kg and 30 kg,respectively, into a pre-mixing tank 10 and homogeneously mixed therein.This mixture is then led into a mixing tank 4 and further mixed andkneaded homogeneously with 60 kg of water supplied from a tank 9, andthe formed solidifying agent is flown into a 200-l drum 5 alreadycontaining the pellets of radioactive waste 7 filled in a wire meshbasket 6. Vacuum deaeration and hardening are accomplished in the sameway as in the preceding embodiment (FIG. 9).

In this way, approximately 480 kg of solidified radioactive waste asshown in FIG. 11 can be obtained. The solidified radioactive wastecaused no deposition of salts on the surface nor leaching of radioactivenuclides, was free of cracks and also had a high strength.

According to these embodiments, it is possible to use either an aqueoussolution or powder of sodium silicate as the solidifying filler by usingcalcium silicate (CaSiO₃) as hardening agent, and there can be obtaineda pellet of solidified radioactive waste which is free of deposition ofsalts or leaching of radioactive nuclides and has excellent waterresistance.

As still another embodiment of this invention, there will be describedbelow, with reference to FIG. 12, a case where not the pelletizedradioactive waste but the radioactive waste (mainly composed of Na₂ SO₄)generated from a nuclear power station is directly treated andsolidified in a 200-l drum. In this case, in order to secure thestrength of the solidified waste and the proper volume reduction ratioof the waste, the radioactive liquid waste contained in a tank 12 isfirst dehydrated and formed into a powder in a dryer 13 and thensupplied into a tank 14. Various methods are known for drying theradioactive liquid waste, such as centrifugal film drying, spray drying,fluidized bed drying, drum drying, freeze drying and crystallization,and any of these methods can be employed in this invention.

After this pretreatment of the radioactive liquid waste, about 200 kg ofa 60 wt% aqueous solution of sodium silicate, about 60 kg of calciumsilicate (CaSiO₃), about 30 kg of cement and about 210 kg of powderedradioactive waste are supplied into a mixing stirrer 4 from theirrespective tanks 1, 2, 3 and 14 and homogeneously stirred and mixed.This mixture is then flown into and filled in a 200-l drum 5. Vacuumdearation is also conducted to eliminate the air bubbles remaining inthe solidifying agent. Thus, by using sodium silicate as filler andcalcium silicate (CaSiO₃) as hardening agent thereof, it is possible tomake a homogeneous, water-resistant solidified radioactive waste asshown in FIG. 13.

An embodiment of this invention will now be described with reference toFIG. 14.

An alkali silicate solution used as the solidifying filler, Portlandcement used as the water absorbent and calcium silicate (CaSiO₃) used asthe hardening agent or durability improver are mixed and this mixture isfilled in the pelletized radioactive waste. The pellet is deaeratedunder a vacuum of below 100 Torr for effecting homogeneous and densefilling. After deaeration, the whole mass is kept under a vacuum ofbelow 40 Torr at 20° C. until the hardening is completed.

According to this embodiment of the invention, the liberated water isurged to evaporate from the alkali silicate solution while the mixedmass is kept under a vacuum of below 40 Torr, and by the time thehardening is completed, the water content is reduced to around 11% toreach an equilibrium with the humidity of the ambient air. Accordingly,the evaporation rate of the liberated water becomes less than 1%/deg⁻¹.It is thus possible to form a sound solidified radioactive waste whichis free of cracks that are injurious to the strength and waterresistance of the solidified body.

The invention has been described regarding some embodiments thereofwhere the radioactive waste (in the form of pellets or liquid) mainlycomposed of sodium sulfate, such waste being generated in boiling waterreactors, is solidified, but the method of this invention can be equallyand as effectively applied to the treatment of radioactive waste mainlycomposed of boron such as one generated in pressurized water reactorsand waste ion exchange resins.

In case of treating the pelletized radioactive waste, the same effectcan be obtained by mixing the pelletized waste with a sodium silicatesolution (or powder of sodium silicate and water), calcium silicate(CaSiO₃) and cement and filling this mixture in a drum, instead ofhaving the drum previously filled with the pelletized radioactive waste.

Also, in the above-described embodiments, the radioactive waste mixtureis filled in a basket 6 placed in the drum 5 so that the pellets ofradioactive waste will not touch the inner wall of the drum, but it isalso possible to attain secure solidification and fixing of thepelletized waste inside the drum by lining the drum with a fibrousmaterial such as glass fiber, asbestos, carbon fiber, or metal fiber.

Further, in the embodiments described above, the air bubbles in thefilled solidifying agent are removed by means of vacuum deaeration, butthe similar effect can be provided by giving vibrations to or heatingthe drum after filled with the solidifying agent.

According to the present invention, it is possible to make a solidifiedradioactive waste which is free of deposition of easily soluble salts onits surface, very scanty in leaching of radioactive nuclides andexcellent in moisture and water resistance, by using a solidifying agentcontaining an alkali silicate or an aqueous solution thereof assolidifying filler.

We claim:
 1. A method for solidifying radioactive waste which comprisesmixing the radioactive waste with a solidifying agent prepared by mixingan alkali metal silicate as a solidifying filler; an inorganic compoundas a hardening agent for reacting with said alkali metal silicate;cement as a water absorbent for absorbing liberated water generated inthe course of the hardening reaction between said alkali metal silicateand said inorganic compound; and water; wherein said inorganic compoundcontains an ion able to bind with an alkali metal in said alkali metalsilicate to form a low-solubility salt,said inorganic compound being acompound of a polyvalent metal ion selected from the group consisting ofCa²⁺, Mg²⁺, Al³⁺, and Fe³⁺ or an H⁺ ion and an ion selected from thegroup consisting of TaO₃ ⁻, AlF₆ ³⁻, NbO₃ ⁻, SiF₆ ²⁻, SiO₃ ²⁻, BeF₄ ²⁻,B₄ O₇ ²⁻, F⁻, IO₄ ⁻, ClO₄ ⁻, BF₄ ⁻ and ReO₄ ⁻.
 2. A method forsolidifying radioactive waste according to claim 1, wherein the contentof said inorganic compound in the solidifying agent is not less than 3%by weight but not greater than 50% by weight.
 3. A method forsolidifying radioactive waste according to claim 1, wherein the contentof said cement in the solidifying agent is not less than 3% by weightbut not greater than 35% by weight.
 4. A method for solidifyingradioactive waste according to claim 1, wherein the content of saidwater in the solidifying agent is not less than 15% by weight but notgreater than 40% by weight.
 5. A method for solidifying radioactivewaste according to claim 1, wherein said alkali metal silicate isselected from the group consisting of sodium silicate, potassiumsilicate and lithium silicate.
 6. A method for solidifying radioactivewaste according to claim 1, wherein said inorganic compound is CaSiO₃.7. A method for solidifying radioactive waste according to claim 1,wherein said alkali metal silicate is sodium silicate.
 8. A method forsolidifying radioactive waste according to claim 7, wherein saidinorganic compound is CaSiO₃.
 9. A method for solidifying radioactivewaste according to claim 2, wherein the content of said alkali metalsilicate as a solidifying filler in the solidifying agent is about 37.5%by weight.
 10. A method for solidifying radioactive waste according toclaim 3, wherein the content of said alkali metal silicate as asolidifying filler in the solidifying agent is about 37.5% by weight.11. A method for solidifying radioactive waste according to claim 4,wherein the content of said alkali metal silicate as a solidifyingfiller in the solidifying agent is about 37.5% by weight.
 12. A methodfor solidifying radioactive waste according to claim 1, wherein theviscosity of the solidifying agent before it is hardened is less thanabout 3,000 cP.
 13. A method for solidifying radioactive waste accordingto claim 1, wherein the radioactive waste and the solidifying agent aremixed in approximately equal amounts by weight.
 14. A method forsolidifying radioactive waste according to claim 1, wherein the contentof said inorganic compound in the solidifying agent is not less than 3%by weight but not greater than 50% by weight; the content of said cementin the solidifying agent is not less than 3% by weight but not greaterthan 35% by weight; the content of said water in the solidifying agentis not less than 15% by weight but no greater than 40% by weight; andthe content of said alkali metal silicate in the solidifying agent isabout 37.5% by weight.
 15. A method for solidifying radioactive wasteaccording to claim 8, wherein the content of said inorganic compound inthe solidifying agent is not less than 3% by weight but not greater than50% by weight; the content of said cement in the solidifying agent isnot less than 3% by weight but not greater than 35% by weight; thecontent of said water in the solidifying agent is not less than 15% byweight but not greater than 40% by weight; and the content of saidalkali metal silicate in the solidifying agent is about 37.5% by weight.